rsieminski

I built one out of a body board for my kids, 4,12,and 13. The 4 y/o gets a kick out of it, but the 12 and 13 y/o need a little more than 1/3 throttle to keep their attention. Any more juice and it porpoises up and down in the front. It goes better in the grass than in the water. I attribute this to the hull, uh I mean body board being totally flat. Sooooo, i'm going to build another, and want hull design ideas. Just a pic will do.
Thanks,
--Rick

gvs

A pict of my hull, just painted...

rsieminski

Is that flat or does it have taper on the bottom?

gvs

it´s flat and made of fiberglas

WA8INZ

How do you post pictures in this forum?. I just scratch build a 43"x18" airboat and would like to see what the boat folk thought about it. Thanks in advance for your help.

gvs

Just use the "Attach file" otion at the end of the post reply page.
There are some limitations about file size and so, you´ll find the info in the post reply page.

brother

Hey rsieminski, Try moving your CG forward; I built an airboat from scratch and made sure it had angled sides. It also does great on grass but I had trouble getting it to come up on plane. I had to move the CG back to take weight off the front. Sounds like your problem is the opposite of mine. Brother

TERBObob

My Feedback: (21)

I also built a scratch built , but plan on using it for the snow . Shes 36 long , 29 wide , 2 1/2 deep , running a 32CC Ryboi swinging a 16/10 3 blade pusher prop . In reguards to your question , I think you should state what type of engine you are using , since this will make a difference in size of boat , and also what you plan on using it for . The same design for water does not do as well on ground , or snow and vice versa .
My friend has 4 airboats he built and all are smaller and all running nitro . ( also smaller props as well )
The largest he has is a 4 cycle 80 which can not keep up with my Ryboi , on the water , or grass . ( FTR - the complete package weighs in at just over 20 pounds whereas his weighs a mere 13 . But then again , he only has that lightweight nitro motor also - LOL )
We have currently only run them on the grass and water , but awaiting the "white" stuff to hit the ground.
Also , if your CG is ok , then try adjusting the attitude of your prop , try lowering the attack angle a bit . ( make the prop angle face more down towards the front of the boat . This will stop the "lift" effect ( porpoising ) that you are getting . Do it a LITTLE at a time till you get her to stay level while full bore .

WA8INZ

I have no pictures of the inside of the hull, what you see on my website is it (Sorry).

Wile E. Coyote

Here is my airboat, O.S. 46 powered.
Great acceleration and good top speed

pro27

Here are some pics of some of my smaller airboats.

pro27

My Dumas Swamp Buggy powered with a MDS .18.

pro27

My Aqua Craft Air Force with a MDS Pro .40.

I have other boats, but no pictures at this time.

rsieminski

Great looking airboats!!!
I'm working on one now, off and on. I'm to the point where I need to put the supports to mount the engine, but I need to know here it should balance. I don't know wheather this is most important in ref to the all flat part or the whole thing. This is going to be a .40 sized hull. Could you guys post where the cg is in reference to the front, and then in ref to the flat part of the hull.
Thanks,
--Rick

rclooney

do airboats scoop air under them to create lift? if so, keep of water!

TERBObob

My Feedback: (21)

CG of any boat is figured on where it balances out at . Doesn't matter about the flat spot or whatever . The CG is for the entire boat .
Scoop air under them to create lift ? no ..... they get on plane just as a normal boat does ( normal meaning a prop driven one ) .
( actually , after seeing a LOT of different air boats , it makes me actually wonder how many out there actually DO get on plane - )
Tomorrow , if it is not too windy out , I will video tape some action of mine and post a link here .
( homemade hull running a Ryboi 32 CC modded to the max , bored carb , swinging a 16x10 3 blade pusher prop )

pro27

I tried a design about 10 years ago that had the motor up front. It was a complete failure.
But then I didn't expect it to work.
Imagine if you will, an airplane fuselage, minus the rudder and stab, attach widely spaced floats
and then add the largest motor you had. It went straight just fine but would not turn worth a damn.

It would be interesting to hear how yours is set up and how well it turns.

pro27

Actually............the CG of an airboat is calculated the same as you would any monohull type boat (with some exceptions). The CG should be 29% of the total HULL length, measured from the transom forward.
Example.......Hull length of 30". 29% of 30" is like about 9.91". From the transom, measure forward 9.91" and this is your theretical CG. Balance out from there.

There are some guys out there that have been building AB's a slightly different way for a while.
They are mounting their motors almost mid hull. This makes a very 'responsive' boat.
It seems that the closer to the hull length's center the more pitch (?) sensitive it becomes, thus more 'steering enabled' it becomes. (this is the exception I parentisised above).

rsieminski

Thanks pro27! Still a few questions though. The transom would be the farthest back portion of the hull? Uh right? The ones with the engine in the middle of the boat, do they still balance out at the 29% mark??

It would seem to me that depending on how far the front of the hull tapers from the top of the deck to the back would come into the equation somehow.

The 2 kindergarden hull pictures should have 2 different balance points, shouldn't they?

Wile E. Coyote

pro27,
I have an old italian magazine which explains how to build an "airboat" with two airplane foam float connected to a tube "fuselage" with air rudder. With an old OS40 reached over 60 km/h and it was also quite stable on turns.
The airboat in my pic it's also very fast perhaps over 70/75 km/h, but it will flip on high speed turns and if the water is not completely calm (i drive it in a lake)

pro27

On mid motor mounted hulls, in my opinion, the CG becomes a grey area. ( this will obviously be up for debate). Most of the hulls I've seen that use this motor position have a rather rounded cross section. More like that of a body board, some are even more eliptical in cross seaction. Check out the following site and you'll see what I mean.
www.geocities.com/rcairboat78

As for the bottom taper, it won't affect the balance point, but rather it does play the major roll in how the hull rides, or planes. Your upper hull drawing will be a much faster hull owing to having less wetted surface than the bottom hull. However......it will not be as stable a hull as the bottom one because it will ride much looser on the water.

This area as well as length vs width , are very debatable. When it comes to airboats, not much study or testing has been done to really knnow what works or why. Airboats are looked at mostly as 'fun'boats, which they are, but can also be very high performance craft.

Unlike other forms of boat racing where a lot of work has been done. Just look at the advancements in OB tunnel hull designs. Thay all look the same form a distance, but look at the bottom hull designs and a lot of tech work has been acomplished over the years.

The CG in relation to the motor placement is a very interesting area.
The CG of almost any hull, mono, hydro, cat, is a math equation of physics, not overall physical size, shape nor displacement. Engine placement does play a role in the equation to an extent. Most mono hulls place the motor at about 30% from the transom. Most hydros, place the motor much farther than that, some at about 55-60%. A big difference.

On an airboat, the motor placement plays a bigger role in the thrust effect it has. The motor thrust lines and angles are what help to achieve the ride attitude of the hull. Having the motor thrust line more in line with the hulls cg (neutral) makes it more sensitive. I always try to imagine the 3 controls of flight. Thrust, yaw and pitch. These play a big role in an airboat more than they do in a conventiona;l boat.

Time to digest...........

Cactus.

My Feedback: (1)

i know my bodge was on the plane, very little wake it it side skipped great in the turns. for a total lash up it went better than expected, i bet some people would pay for performace like that
but alas now it's in bits, the bodges finaly gave way. time to make a propper one. bet it wont work as well

TERBObob

My Feedback: (21)

Just a short little vid of running my home-built airboat today ( was a bit too windy , as can be seen by the brush on the side of the lake .)
File is a little large ( 7 megs ) , but it will give you an idea as to how the boat handles with figuring the CG the same as a regular style boat ( since that was all I had to go by )

http://home.mchsi.com/~terboinc/Airboat.wmv

([8D] I left the "little" scrunch in there on purpose ... kind of shows you how the wind took control of the airboat - ) If you listen closely , you can me get a little upset along with the HARD sound of the clunk , that the boat did when she hit that ole rock . ( no damageT.G. )
And FTR - she was NEVER over half throttle
Oh yeah .... this boat was not even built for the water , but rather for me to have fun with on the snow and ice .

pro27

I couldn't quite make out the words you said in the vid. Could you repeat them?

Is that the normal body of water you run it on? Seems rather a small area. Where I run looks like the ocean compared to it.

Its a good thing there was no damage.
What are the main construction mtls you used?

pro27

Do you have any pics of the underside?
What are the dimensions?
power is a 40 or 46 LA?